Maintaining the adsorption bed on feed until the Xe concentration at the outlet of the bed is greater than or equal to 90% of the inlet concentration, enables the process to be operated under conditions wherein oxygen can be used safely as a purge fluid for the temperature swing adsorption process. Moreover in this way, the bed can be operated under pressure and temperature conditions where other gases can co-adsorb. This serves to extend the range of process conditions useable for the inventive process, versus those of the prior art.
In U.S. Pat. No. 5,039,500 to Shino et al, as an example, discloses a process for producing high purity xenon with liquid oxygen from a main condenser of an air separation unit. In the process of Shino et al, the liquid oxygen stream, containing xenon, krypton and hydrocarbons is first gasified before being contacted with an adsorbent at a preselected temperature and pressure so as to adsorb on the adsorbent xenon, but not oxygen, krypton or hydrocarbons contained in the oxygen stream. The adsorbent is regenerated using a purge gas and by heating. The basic adsorption process can be augmented by other unit operations to increase the purity of the xenon, including a solid-gas separating column, a catalyst column, a moisture and CO2 removal column and so on. A disadvantage of this process is that the liquid feed stream from the cryogenic plant must be converted to a gas stream prior to being contacted with the adsorbent bed. Moreover from claim 1, the adsorption process must be operated under preselected temperature and pressure conditions such that xenon is adsorbed and the krypton, hydrocarbon and oxygen are not. This places limitations on the operating conditions for the process. From embodiment 1 (column 3, lines 20-48) however, it appears this approach of identifying pressure and temperature conditions, where xenon is adsorbed and oxygen, krypton and hydrocarbons are not, was only partially successful. In this embodiment, a silica gel adsorbent was contacted until Xe breakthrough was achieved with a gasified stream at −170° C. containing 31 ppm xenon, 70 ppm krypton and 38 ppm methane and low concentrations of other hydrocarbons in an oxygen matrix. After the gas was heated to 120° C. to regenerate the adsorbent, the effluent concentrations became 1.4% xenon, 0.14% krypton, 0.066% hydrocarbon and balance oxygen. The fact that the krypton, and hydrocarbons became significantly enriched beyond their feed concentration, in the same way as the xenon, suggests that these components were also adsorbed under the conditions used for the adsorption feed step, in an apparent contradiction with the process as claimed. It is likely therefore that due to this co-adsorption of other components, especially the hydrocarbon components and their enrichment by adsorption, that later embodiments employ a catalyst column to remove these hydrocarbons and a CO2 and moisture removal column thereafter, to remove the products of hydrocarbon combustion over the catalyst. By contrast, in the present adsorption process the feed stream can be liquid or gas phase and during the purging and heating steps, employed to recover the xenon product, the concentration of krypton and hydrocarbons is much less than their concentrations in the feed. In the art of Shino et al, as clarified in embodiment 1, 38 ppm methane and low concentrations of other hydrocarbons in the feed, became 0.066% or 660 ppm during the heating step used to regenerate the adsorbent bed. This is an enrichment of approximately 17 times the concentration of hydrocarbons in the feed.
U.S. Pat. No. 4,874,592 also to Shino et al, discloses an adsorption-desorption process, wherein xenon is concentrated from a vented liquid oxygen stream by successive stages of adsorption and desorption and wherein the hydrocarbons are removed catalytically from the xenon gas stream recovered after the first adsorption stage. From examples 1 and 2 and as shown in FIGS. 1 and 2 of U.S. Pat. No. 4,874,592, the vented rare gas containing stream from a rectifying column, is introduced into a first adsorption column, where a silica gel adsorbent, capable of selectively adsorbing the xenon is allowed to saturate. The product stream from this first adsorption column is collected by reducing the pressure and by heating the column. The product stream contained a mixture of xenon, krypton and hydrocarbons in enriched concentrations above the feed composition. A catalyst unit operation and subsequent carbon dioxide and water removal column were used for hydrocarbon removal, prior to a second adsorption column, employed to increase the purity of the rare gas products still further. As clarified in Example 1 of this patent, venting the liquid oxygen stream generates a gaseous oxygen feed stream to the adsorption system. As described above, the adsorption process of the present invention is compatible with either a liquid or a gas feed and therefore the step of venting the liquid oxygen is not required. Moreover, the present process is operated such that enrichment of the hydrocarbons beyond their concentration in the feed stream is avoided, and therefore a step of catalytic oxidation, as described in the prior art is not required.
U.S. Pat. No. 6,658,894 to Golden et al, discloses a process of recovering at least one of xenon or krypton from and oxygen-containing gas stream by selectively adsorbing the xenon and/or krypton using a zeolite of type X exchanged with Li and Ag. According to Example 7, which showcases the key steps in the process of Golden et al, a liquid oxygen stream containing 17 ppm xenon, 95 ppm methane, and 10 ppm nitrous oxide was passed through a silica gel bed wherein the nitrous oxide was removed. The nitrous oxide free effluent was vaporized to 113 K and a portion of this gas stream was sent to a bed containing the type X zeolite exchanged with lithium and silver. The breakthrough of methane was detected after 190 minutes on stream, whereas after 1400 minutes on stream, no breakthrough of xenon had occurred. At this point, the feed step was stopped and regeneration was started using a nitrogen purge gas at 113 K. From the data in FIG. 4 of the Golden et al patent, the methane concentration during desorption increased to a maximum of between 8000-9000 ppm. The xenon product was collected by further warming the adsorption bed. Key features of the art of Golden et al are:                Use of the type X zeolite exchanged with both Li and Ag.        Operation of the adsorption process to the point where breakthrough of xenon was not observed.        Desorption under nitrogen, wherein the methane concentration at the outlet significantly exceeds the methane concentration in the feed stream (95 ppm versus 8000 to 9000 ppm).        
In the process of the present invention, the methane levels during desorption do not show this enrichment behavior observed in the data presented in Golden et al. Moreover, the present process can be operated in a liquid phase and a Li and Ag type X zeolite is not required.
U.S. Pat. No. 3,971,640 to Golovko discloses an adsorption process for recovering a krypton-xenon concentrate from an air stream. In the process of Golovko, a gaseous air stream at 90-110K containing admixtures of krypton, xenon and hydrocarbons is passed through an adsorbent having pore-openings from 5-150 Å, during which time the krypton, xenon, nitrogen, oxygen and hydrocarbons are adsorbed. The feed step is ended when krypton is detected at the outlet of the adsorber. At this point, a staged temperature desorption wherein the temperature is raised firstly from 90-110 K to 250-280 K wherein xenon, krypton, oxygen, nitrogen and hydrocarbons are desorbed from the adsorbent and thereafter the bed is heated further from 250-280 K to 500-650 K with the desorbed products at this point discarded to atmosphere. Unlike the process of Golovko et al, during the desorption step of the process of the invention, substantially only xenon and oxygen are desorbed with any additional components, such as hydrocarbons, are desorbed at concentrations significantly less than their concentration in the feed stream. From example 2 in Golovko, hydrocarbon levels of 2% were measured during desorption which again suggests significant adsorption and concentration of these hydrocarbons by adsorption which does not take place in our process. Moreover in the present process, the use of staged desorption temperatures up to 500-650 K is not required.